With recent advances in downsizing and weight reduction of semiconductor LSIs, chip components and the like, downsizing and weight reduction of wiring boards on which such elements are mounted are also desired. To meet such a request, multilayer ceramic wiring boards that allow high-density wiring thereon and can be formed thin have become increasingly valued in today's electronics industry.
Methods of manufacturing a multilayer ceramic wiring board are roughly classified into two kinds. One method is called multilayer printing method, in which insulating layers and conductive layers are printed alternately on a ceramic substrate. Another is called laminating method, in which a plurality of green sheets are laminated and fired.
The multilayer printing method has a problem of short circuits caused by involving foreign matters such as a little piece of thread during printing. To solve this problem, repeated printing of insulating paste to thicken insulating layers is performed. However, the operation becomes more complicated and chances of foreign matters involving increase when printing and drying are repeated. Therefore, this is not a complete solution.
Another solution is using a metal mask or low mesh screen mask to obtain a thick film at one printing operation. However, this method has problems such as occurrence of deviations in film thickness and difficulty of high-density wiring.
On the other hand, with the lamination method, shrinkage of the multilayer ceramic wiring board is involved by a sintering when the laminate is fired. The shrinkage involved by the sintering vary with board materials used, compositions of green sheets, lots of particles, and the like. The shrinkage poses several problems in production of a multilayer wiring board.
Firstly, because inner layer wiring in a green sheet laminate are fired before forming wiring on a uppermost layer, a high degree of plane-directional shrinkage of the board material hinders connection between patterns on the uppermost wiring and electrodes on the inner layer. As a result, a land of unnecessarily large area must be formed for electrodes on the uppermost layer so as to allow the shrinkage error. Therefore, such lamination method is difficult to be used for high-density wiring. To solve this problem, in some cases, a large number of screen masks for the wiring on the uppermost layer are prepared according to degrees of shrinkage and used according to shrinkage rates of the wiring board. This solution requires a large number of screen masks and is uneconomical.
If the wiring on the uppermost layer are fired with the inner layer at the same time, such a large land is unnecessary. However, with this simultaneous firing method, shrinkage of the wiring board still exists. As a result, in some cases, cream solder cannot be printed on required positions of the multilayer ceramic wiring board, in printing the cream solder for mounting components thereon. In addition, when components are mounted on the wiring board, the shrinkage causes displacement between the components and their predetermined positions.
In order to minimize such shrinkage error, it is necessary to sufficiently control not only substrate materials and green sheet compositions but also difference in particle lots and lamination conditions (press pressures and temperatures) during manufacturing process. However, it is said that deviation of approx. ±0.5% in shrinkage exists.
This problem resulting from shrinkage is not only of multilayer wiring boards but also is common to sintering of ceramics and glass ceramics.
Japanese Patent Laid-Open Publication No. H05-102666 discloses the following method. When a low-temperature sintering glass ceramic laminate is fired, a green sheet including inorganic fine particles that do not sinter at a sintering temperature of glass ceramic low-temperature sintering material is attached to at least one face of the low-temperature sintering glass ceramic laminate In the following description, “sintering” means not only simple sintering of crystals but also include a binding by a melting of glass components. In this method, after the laminate is fired, the inorganic fine particles are removed. As a result, the low-temperature sintering materials are sintered only in the thickness direction and a wiring board having no plane-directional shrinkage is produced. Consequently, the above problem resulting from the shrinkage of the wiring board involved by firing can be solved.
However, the above method for producing a wiring board requires a green sheet of inorganic fine particles other than glass ceramic green sheets. In addition, a process of removing non-sintering inorganic fine particles from the wiring board after firing is necessary.